The present invention relates to the field of voltage regulation, and, more particularly, to voltage regulation for providing a constant supply voltage from a variable input voltage.
Voltage regulators are used very widely in supply circuits. Voltage regulators make it possible to provide supply voltages with low power losses. When the energy source is a battery, it is important for the consumption of the voltage regulator or regulators to be low. This is especially important when the batteries of mobile telephones have small dimensions and whether the mobile telephone is in active mode or in standby mode.
In general, the performance of voltage regulators, in terms of output current consumption, is guaranteed down to a minimum battery voltage above their output voltage. The difference between the minimum voltage of the battery and the output voltage of the regulators is often referred to as the drop voltage, denoted Vdrop.
In the case of a battery voltage, which is below the abovementioned threshold, many regulators are disabled. However, in the case of mobile telephones, the voltage regulators, which supply the real-time clock part, may remain operationally active with degraded performance. In this case, the output voltage of the regulator is imposed by the supply, and may therefore no longer reach the desired voltage. Thus, the feedback loop of the voltage regulator can no longer follow the preset. The regulator goes out of balance, thereby creating a debiasing of certain transistors and an over consumption in the comparison-amplification stage.
The invention provides an improved voltage regulator. The invention provides regulation adapted to a low battery voltage that induces no over consumption, is economical to produce and can be easily integrated. The invention provides a voltage regulator having low consumption regardless of the input voltage.
The voltage regulating device, according to an aspect of the invention, comprises a means of comparison of a voltage proportional to an output voltage and of a fixed reference voltage, the fixed reference voltage being received on a first input and the voltage proportional to an output voltage being received on a second input, and a variable resistance-forming means controlled by the output of the means of comparison and disposed in such a way that the output voltage remains substantially constant.
The device is supplied with an input voltage, which may vary. The device comprises another means of a voltage proportional to the input voltage and of the fixed reference voltage. The device comprises a switching means controlled by the output of the other means of comparison so as to send the fixed reference voltage to the first input of the means of comparison if the fixed reference voltage is less than the voltage proportional to the input voltage, and to send the voltage proportional to the input voltage to the first input of the means of comparison if the fixed reference voltage is greater than the voltage proportional to the input voltage, in such a way that the output voltage remains substantially constant if the input voltage is greater than a threshold, and substantially equal to the input voltage if the input voltage is less than the threshold.
In one embodiment of the invention, the means of comparison comprises an amplifier-comparator and the variable resistance-forming means comprises a MOS transistor. In one embodiment of the invention, the device comprises two resistors in series between the output of the variable resistance-forming means and a ground, the voltage proportional to an output voltage being tapped off at the point common to the two resistors.
Preferably, the other means of comparison comprises a hysteresis-type comparator. In one embodiment of the invention, the device comprises two resistors in series between the input voltage and a ground, the voltage proportional to an input voltage being tapped off at the point common to the two resistors. Preferably, the switching means comprises two breakers, each controlled by the output of the other means of comparison, the first able to set the first input of the means of comparison to the fixed reference voltage Vref, the second able to set the first input of the means of comparison to the voltage Vpve proportional to the input voltage Ve.
In one embodiment of the invention, the first breaker is on and the second breaker is off if the fixed reference voltage is less than the voltage proportional to the input voltage. The first breaker is off and the second breaker is on if the fixed reference voltage is greater than the voltage proportional to the input voltage. In one embodiment of the invention, the device is associated with a supply battery. The invention also relates to a portable system of the mobile telephone type comprising a device as described above.
The invention also provides a voltage regulating process, comprising steps of comparing a voltage proportional to an input voltage which may vary and of a fixed reference voltage, fixing a comparison voltage equal to the fixed reference voltage if the fixed reference voltage is less than the voltage proportional to the input voltage and equal to the voltage proportional to the input voltage if the fixed reference voltage is greater than the voltage proportional to the input voltage. The steps may further include comparing the comparison voltage and a voltage proportional to an output voltage, and varying the value of a resistor as a function of the comparison in such a way that the output voltage remains substantially constant if the input voltage is greater than a threshold, and substantially equal to the input voltage if the input voltage is less than the threshold.
Stated otherwise, during normal operation, the regulated voltage is compared with a fixed reference voltage of the circuit. From the moment that the input voltage of the regulator is too low, the preset can no longer be reached and the output voltage will fall and cause the system to go out of balance. The invention makes it possible to retain a single bias while preserving the balance of the system. To do this, the value of the minimum permitted voltage supply is detected and, from this instant onwards, the present invention uses a reference proportional to the supply rather than a fixed reference. The proportional reference will decrease with the supply and the regulation may therefore be preserved and over consumption avoided.
To detect of the mode of low supply voltage, also referred to as drop-out, a hysteresis-type comparator compares the fixed reference voltage Vref with a fraction of the supply voltage Ve. If Vref greater than kxc3x97Ve, then the present invention switches to drop-out mode. Conversely, if Vref less than kxc3x97Ve, then the present invention switches to normal mode. The output of the hysteresis-type comparator will make it possible to control two breakers, which will send the fixed reference voltage Vref or the variable voltage Vpve to the input of a comparator to serve as the reference voltage.